Exhaust emission control system, method of calculating pressure loss of filter, and method of manufacturing filter

ABSTRACT

There are disclosed an exhaust emission control system including an internal combustion engine, a filter, and an exhaust pipe, a method of manufacturing a filter suitable for the system, and a method of calculating a pressure loss. The system comprises combustion means for intermittently burning particulate matters arrested by the filter, the filter includes porous partition walls extending from one end face to the other end face thereof, and a large number of through channels partitioned by the partition walls, predetermined through channels are sealed at either of the end faces, and assuming that a partition wall thickness is (X) μm, and the number of through channels per unit area in a cross section vertical to a longitudinal direction of the through channels is (Y) cells/cm 2 , X and Y come within a range surrounded by straight lines connecting points A 1 , B 1 , C 1 , and D 1  in this order in FIG.  1  in the exhaust emission control system. There are provided the exhaust emission control system, the method of manufacturing the filter, and the method of calculating the pressure loss by the filter with good accuracy in which a temperature gradient produced in the filter at the time of regeneration is suppressed while suppressing a rise of the pressure loss by the filter and which are accordingly superior in reliability.

TECHNICAL FIELD

The present invention relates to an exhaust emission control system, amethod of calculating a pressure loss by a filter, and a method ofmanufacturing a filter, particularly to an exhaust emission controlsystem in which a pressure loss is small and a temperature gradientgenerated in a filter at the time of combustion is small, a calculationmethod capable of calculating a pressure loss by a filter in an exhaustemission control system with good accuracy, and a method ofmanufacturing a filter, in which an appropriate filter shape can beeasily determined.

BACKGROUND ART

As a conventional exhaust emission control system for efficientlyremoving particulate matters (hereinafter referred to as PM) exhaustedfrom an internal combustion engine, a system using a diesel particulatefilter (hereinafter referred to as DPF) has heretofore been proposed.Unless the captured PM is removed in the system using the DPF, thefilter is finally clogged, and therefore the filter needs to beperiodically regenerated.

In general, the regeneration of the filter is possible, when the DPF isheated and burnt. However, for example, an exhaust gas temperature of adiesel engine does not easily reach to a burning temperature of the PM.Therefore, there has been considered a method in which the temperatureof the DPF is raised by external heat sources such as an electric heaterand a burner to burn soot that is a main component of the PM, or amethod in which the DPF is periodically replaced, and the removed DPF isheated by an electric furnace.

However, in the method in which the filter is heated by the externalheat sources such as the electric heater and the burner, the PMdeposited on the filter can be comparatively stably burnt, but theelectric heater or the combustion burner is a complicated/expensivedevice, and therefore this method has been restricted to some specialapplications. In the method in which the DPF is periodically replaced,handling of the filter has been troublesome.

To solve the problem, there has been proposed a system in which a timeto supply fuel to an internal combustion engine, for example, a fuelinjection time in a diesel engine or the like is adjusted to raise anexhaust gas temperature, and the PM deposited on the filter isperiodically burnt. This system is sometimes combined with a method forlowering the burning temperature of the PM in order to more sufficientlyburn the PM. Since the method is a comparatively simple method, it hasalso been possible to mount an exhaust emission control system in whichthe DPF is used in removing the PM exhausted from the diesel enginemounted on the automobile.

However, in the method in which the PM is deposited on the filter, andthe injection time of the diesel engine is adjusted at a certain timeinterval to raise the exhaust gas temperature, the exhaust gastemperature rises in a remarkably short time. Therefore, as comparedwith the case where the PM is burnt by the electric furnace or the like,a rapid temperature rise in the filter at the time of regenerationeasily occurs, additionally a temperature gradient in the filterincreases, and there has been a possibility that the filter isdisadvantageously cracked. Especially in the method for lowering theburning temperature of the PM, activation energy of the PM is lowered.Therefore, as compared with a case where the PM is simply burnt by theexhaust gas temperature rise from the engine, the PM is burnt in acomparatively short time, thermal energy received by the DPF per unittime increases, an excessive temperature distribution is made in theDPF, and there has been a possibility that the DPF is more easilycracked or that materials are molten.

On the other hand, the filter has a mechanism in which openings of cellsof a honeycomb structure constituted of a porous ceramic are alternatelysealed, and the PM is captured during passing of the exhaust gas throughporous honeycomb walls. During the passing of the exhaust gas throughthe walls, emission resistance is generated, and there is a problem thata difference in pressure loss before/after the filter increases. Whenthe pressure difference increases, an output from the engine drops.Therefore, to obtain the same performance, as compared with a case wherethe filter is not mounted, much more fuel is required. When the pressuredifference excessively increases, there is a possibility that the fueldoes not burn well in the engine and the engine does not operatedisadvantageously. Therefore, it is an important function of the filterto reduce the pressure loss difference before/after the filter.

Moreover, when a fuel auxiliary agent is used for lowering the burningtemperature of the PM, and when the fuel auxiliary agent burns togetherwith the PM, a large amount of ashes are generated, and there is aproblem that a back pressure before/after the filter increases. Even ina method in which a catalyst is imparted to the filter, the catalystitself or a washed coating of the catalyst closes pores in the filter,and therefore there is a problem that the pressure loss in the filterincreases. Therefore, in the exhaust emission control system, it hasbeen necessary to suppress the increase of the pressure loss to theutmost while well suppressing a filter temperature at the time ofregeneration.

On the other hand, various methods of calculation of the pressure lossof the filter have heretofore been proposed, but the pressure loss canbe calculated well on conditions that any PM is not deposited on thefilter, but it has not been possible to calculate the pressure loss in acase where the PM is deposited. Therefore, since it is difficult tocalculate the pressure loss of the filter at the time of actual use inthe exhaust emission control system with good accuracy, it is difficultto predict an optimum filter structure in which the pressure loss iswell balanced with the filter temperature at the time of regeneration.There has been a demand for a method of calculating the pressure losswith good accuracy in order to obtain the exhaust emission controlsystem provided with the above-described optimum filter.

DISCLOSURE OF THE INVENTION

The present invention has been developed in consideration ofcircumstances, and an object is to provide an exhaust emission controlsystem in which a rise of a pressure loss by a filter is suppressedwhile suppressing a temperature gradient generated in the filter at thetime of regeneration and which is accordingly superior in reliability.Another object of the present invention is to provide a method ofmeasuring a pressure loss by a filter with good accuracy. Still anotherobject of the present invention is to provide a method of manufacturinga filter, capable of easily determining an appropriate shape of thefilter preferably usable in the above-described exhaust emission controlsystem.

According to the present invention, there is first provided an exhaustemission control system including: an internal combustion engine; afilter for capturing particulate matters in an exhaust gas exhaustedfrom the internal combustion engine; and an exhaust pipe for introducingthe exhaust gas into the filter, the system comprising a combustionmeans or device for intermittently burning the particulate matterscaptured by the filter, the filter being a honeycomb filter including:at least two end faces; porous partition walls extending from one endface to the other end face; and a large number of through channelspartitioned by the partition walls and extending from one end facethrough the other end face, predetermined through channels being sealedin one end face, remaining predetermined through channels being sealedin the other end face, wherein assuming that a partition wall thicknessis (X) μm, and the number of through channels per unit area in a sectionvertical to a longitudinal direction of the through channels is (Y)cells/cm², X and Y fall within a range surrounded by straight linesconnecting points A1 (X is 267, Y is 50.4), B1 (X is 343, Y is 27.1), C1(X is 470, Y is 27.1), and D1 (X is 394, Y is 50.4) in this order inFIG. 1.

In the first aspect of the present invention, the X and Y preferablyfall within a range surrounded by straight lines connecting points A2 (Xis 305, Y is 46.5), B2 (X is 356, Y is 31.0), C2 (X is 432, Y is 31.0),and D2 (X is 381, Y is 46.5) in this order in FIG. 1, and the X and Yfurther preferably fall within a range surrounded by straight linesconnecting points A3 (X is 330, Y is 42.7), B3 (X is 356, Y is 34.9), C3(X is 406, Y is 34.9), and D3 (X is 381, Y is 42.7) in this order inFIG. 1. The internal combustion engine is preferably a diesel engine.The combustion means or device preferably includes an exhaust gastemperature raising means or device for raising a temperature of theexhaust gas in such a manner as to start the burning of the particulatematters captured by the filter, and the exhaust gas temperature raisingmeans or device further preferably includes an adjustment device foradjusting a time to supply fuel to the internal combustion engine. Theexhaust gas temperature raising means or device also preferably includesa supply device for supplying the fuel into the exhaust pipe. Theexhaust emission control system preferably further includes a means ordevice for lowering the burning temperature of the particulate matterscaptured by the filter, and the exhaust emission control system alsopreferably includes a means, matter, or device for promoting the burningof the particulate matters captured by the filter. The filter preferablycontains a ceramic material as a main component, and the filter is alsopreferably constituted of a plurality of integrated segments of ahoneycomb structure.

According to the present invention, there is secondly provided a methodof calculating a pressure loss of a honeycomb filter including: at leasttwo end faces; porous partition walls extending from one end face to theother end face; and a large number of through channels partitioned bythe partition walls and extending from one end face through the otherend face, predetermined through channels being plugged in one end face,remaining predetermined through channels being plugged in the other endface, the method comprising the steps of: decomposing the pressure lossinto at least a pressure loss in a plugged portion, a pressure loss inthe through channel, and a pressure loss in the partition wall; anddecomposing the pressure loss in the partition wall into pressure lossesin cases where any particulate matter is not deposited in the filter andwhere the particulate matters are deposited to calculate the pressureloss. In the second aspect of the present invention, the methodpreferably comprises the steps of: measuring the pressure loss in thecase where the particulate matters are deposited in the predeterminedfilter; and calculating the pressure loss in the partition wall in thecase where the particulate matters are deposited in the filter based onan equation obtained by curve fitting of an increase behavior of theobtained pressure loss.

According to the present invention, there is thirdly provided a methodof manufacturing a filter, wherein a shape of the filter is determinedby use of a pressure loss value obtained by the calculation methodaccording to the second aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing ranges of a partition wall thickness and acell density in a filter according to the present invention;

FIGS. 2(a) and (b) are diagrams schematically showing one configurationof the filter according to the present invention, FIG. 2(a) is aperspective view, and FIG. 2(b) is a partially enlarged view;

FIG. 3 is a diagram schematically showing a constitution of an exhaustemission control system according to the present invention;

FIG. 4 is a diagram schematically showing a method of measuring aplugging pressure loss of the filter according to the present invention;

FIG. 5 is a diagram schematically showing a method of measuring a wallpassage pressure loss according to the present invention;

FIG. 6 is a schematic plan view showing another configuration of thefilter according to the present invention;

FIG. 7 is a graph showing results obtained by a method of calculating apressure loss according to the present invention;

FIG. 8 is a graph in which the results obtained by the method ofcalculating the pressure loss according to the present invention arecompared with actual measurement;

FIG. 9 is a graph showing results of measurement of a temperaturegradient of the filter in an example.

BEST MODE FOR CARRYING OUT THE INVENTION

An exhaust emission control system according to a first aspect of thepresent invention, a method of calculating a pressure loss by a filterin the exhaust emission control system according to a second aspect, anda method of manufacturing a filter according to a third aspect will bedescribed hereinafter with reference to the drawings, but the presentinvention is not limited to the following embodiment. It is to be notedthat in the following, a section means a section vertical to alongitudinal direction (X-axis direction in FIG. 2(a)) of a throughchannel unless otherwise mentioned.

First, a method of calculating a pressure loss of a filter according tothe second aspect of the present invention will be described.

In an example of the filter according to the second aspect of thepresent invention, as shown in FIGS. 2(a), (b), the filter includes atleast two end faces 42 and 44, porous partition walls 2 extending fromone end face 42 to the other end face 44, and through channels 3 a and 3b partitioned by the partition walls 2 and extending from one end face42 through the other end face 44. The predetermined through channels 3 aare plugged in one end face 42, and the remaining predetermined throughchannels 3 b are plugged in the other end face 44 (not shown).

In this constitution, when the filter is used as a filter 1 forpurifying an exhaust gas from an internal combustion engine 50 as shownin FIG. 3, and, for example, when the exhaust gas is allowed to flowinto the end face 42, the exhaust gas flows into the filter from thethrough channels 3 b opened in the end face 42, passes through theporous partition walls 2, and is exhausted from the through channels 3 aopened in the end face 44. In this case, the partition walls 2 functionas the filter, and, for example, the PM and the like exhausted from adiesel engine can be captured and removed in the filter.

According to the second aspect of the present invention, there isprovided a method of calculating a pressure loss at a time when theexhaust gas passes through the above-described filter. The pressure lossis decomposed into at least a pressure loss in a plugged portion(hereinafter referred to as the plugging pressure loss), a pressure lossin the through channel (hereinafter referred to as the in-channelpressure loss), and a pressure loss during passage of the gas throughthe partition walls (hereinafter referred to as the wall passagepressure loss), and the wall passage pressure loss is decomposed into apressure loss in a case where any PM is not deposited in the filter(hereinafter referred to as the initial wall passage pressure loss) anda pressure loss in a case where the PM is deposited (hereinafterreferred to as the PM pressure loss) to calculate the pressure loss,which is regarded as important characteristics. When the pressure lossis decomposed into factors as described above, and constants in apressure loss theoretical formula are experimentally obtained withrespect to the respective factors, the pressure loss of the filter atthe time of actual use can be calculated with good accuracy.

The pressure loss by the filter in the exhaust emission control systemis decomposed as described above, and can accordingly be represented byEquation (1).ΔP=ΔPm+ΔPc+ΔPwc+ΔPws  (1)

In the Equation (1), ΔP denotes a value of the pressure loss by thefilter, ΔPm denotes a value of the plugging pressure loss, ΔPc denotes avalue of the in-channel pressure loss, ΔPwc denotes a value of theinitial wall passage pressure loss, and ΔPws denotes a value of the PMpressure loss, respectively.

The plugging pressure loss is considered to be caused by contraction andexpansion accompanying a change of a flow path sectional area of anexhaust gas in a plugged portion, and can be obtained, when the pluggedportions in the filter 1 are cut from the filter 1 as shown in FIG. 4,and the pressure loss of only the plugged portion is measured. Thepressure loss can be measured at various flow rates with respect to thefilters having various partition wall thicknesses and any number ofthrough channels per unit section area (hereinafter referred to as thecell density). The plugging pressure loss can be represented byEquations (2), (3) using a pressure coefficient ξ which can be obtainedfrom experiments.ΔPm=ξρv^(α)  (2)ξ=C ₁(1−OFA)^(D1)  (3)

In the Equation (2), ρ denotes a density of the exhaust gas, v denotes aflow rate of the exhaust gas, and α denotes an index which can beexperimentally obtained. In the Equation (3), C₁, D1 denote acoefficient and an index which can be experimentally obtained, and OFAdenotes an opening area ratio of the through channels to a total area.

Since the exhaust gas is considered to constitute a laminar flow whenpassing through the through channels, ΔPc is generally proportional toviscosity, rate, and flow length of the exhaust gas, inverselyproportional to square of a hydraulic diameter, and can be representedby Equation (4). The in-channel pressure loss can be measured using ahoneycomb structure which does not include any plugged portion.ΔPc=C ₂ v ^(m) Lμ/(OFA)/(DH)²  (4)

In Equation (4), C₂ and m denote a coefficient and an index which can beobtained by the experiments, respectively, v denotes the flow rate ofthe exhaust gas, μ denotes the viscosity of the exhaust gas, L denotes alength of the through channel, OFA denotes the opening area ratio of thethrough channels to the total area, and DH denotes the hydraulicdiameter.

It is considered that Darcy's law can be applied to the wall passagepressure loss. A degree of gas transmission of only the partition wallin a state in which any PM is not deposited can be measured, when thepartition wall 2 is set onto a device shown in FIG. 5, and the degreecan be represented by Equation (5).(Degree of gas transmission)={(flow rate of gas)×(partition wallthickness)×(viscosity of gas)}/(pressure loss)  (5)

Here, for example, when a relation between a PM deposited amount perunit filter area of the partition wall in the DPF and the pressure lossof the whole DPF is measured, and an obtained increase behavior of thepressure loss is curve-fitted as a function of a deposited ratio of thePM, a relation between the increase of the pressure loss and thedeposited amount of the PM can be obtained. Here, a curve-fitting methodis not especially restricted, and a general method of recurrent analysiscan be used. Moreover, the pressure loss at a time when the gas passesthrough the partition wall and a PM deposited layer can be representedby Equation (6). $\begin{matrix}\begin{matrix}{{\Delta\quad{Pw}} = {{\Delta\quad{Pwc}} + {\Delta\quad{Pws}}}} \\{= {{\left( {t_{0}/k_{0}} \right)u\quad\mu} + {{f\left( t_{p} \right)}u\quad{\mu/\left( {u_{0}\mu_{0}} \right)}}}}\end{matrix} & (6)\end{matrix}$

In the Equation (6),1/(μ₀μ₀)=6.3×10⁶,

-   -   k denotes an apparent degree (m²) of gas transmission at the        time when the gas passes through the partition wall and the PM        layer,    -   t denotes a total thickness (m) of the partition wall and the PM        layer deposited on the partition wall,    -   t_(p) denotes an apparent PM layer thickness (m),    -   k₀ denotes the degree (m²) of gas transmission of only the        partition wall;    -   t₀ denotes a partition wall thickness (m),    -   u denotes a flow rate (m/s) of the gas which passes through the        partition wall,    -   μ denotes a viscosity (Pas) of the gas    -   μ₀ denotes a standard flow rate (m/s) of the gas,    -   μ₀ denotes a viscosity (Pas) of the gas in a standard state; and    -   f(t_(p)) denotes an increase of the pressure loss accompanying        the increase of the deposited amount of the PM under conditions        of μ₀ and μ₀, respectively.

In the second aspect of the present invention, the pressure loss is notattributed to the filter itself, but the pressure loss attributed to theexhaust pipe connected to the filter is also preferably taken intoconsideration. The pressure loss is considered to be caused by thecontraction and expansion of the exhaust gas, and can be represented byEquation (7).ΔPd=2ρv ₁ ²(1−d ₁ ² /D ²)²  (7)

In Equation (7), ρ denotes the viscosity of the gas, v₁ denotes theexhaust gas flow rate (m/s) in the exhaust pipe, d₁ denotes a diameter(m) of the exhaust pipe, and D denotes a diameter (m) of the filter,respectively.

When the pressure loss is taken into consideration, the pressure lossattributed to the filter can be represented by Equation (8).$\begin{matrix}\begin{matrix}{{\Delta\quad{P\left( {{including}\quad{the}\quad{exhaust}\quad{pipe}} \right)}} = {\Delta\quad P}} \\{= {{\Delta\quad{Pm}} + {\Delta\quad{Pc}} + {\Delta\quad{Pwc}} +}} \\{{\Delta\quad{Pws}} + {\Delta\quad{Pd}}}\end{matrix} & (8)\end{matrix}$

When the pressure loss by the filter is decomposed into at least fourfactors, further the pressure loss by the exhaust pipe connected to thefilter is preferably added, and the coefficients are derived from therespective experimental results, the pressure loss by the filter can becalculated with better accuracy. In accordance with an example of FilterA in which segments each having a basic sectional shape of a 35 mm×35 mmsquare are integrated to form a cylindrical shape having a diameter of144 mm×a length of 245 mm and which has the following filtercharacteristics, a relation among the pressure loss, the cell density,and the partition wall thickness is calculated on the followingcalculation conditions based on Equation (8) and is shown in FIG. 7.

(Characteristics of Filter A)

-   -   Material: silicon carbide    -   Porosity: 46%    -   Average pore diameter: 18 μm    -   Thermal conductivity: 25 W/mK

(Calculation Conditions)

-   -   PM deposited amount per unit volume of the filter:        -   5 kg/m³    -   Exhaust gas flow rate: 10 Nm³/min.    -   Temperature: 665° C.

It has heretofore been considered that the pressure loss uniquely drops,when the cell density is increased in order to increase a filter area.However, by this analysis, it has been found that with the equalpartition wall thickness, a pressure loss value is minimized in thevicinity of a cell density of about 39 cells/cm², and the pressure lossincreases, when the cell density is not less than the value. This isconsidered to be attributed to the increase of the in-channel pressureloss by a decrease of the hydraulic diameter. It has also been confirmedthat the smaller the partition wall thickness is, the wall passagepressure loss drops.

Next, a method of manufacturing a filter according to a third aspect ofthe present invention will be described. According to the second aspectof the present invention, the pressure loss in the case where the PM isdeposited can be calculated with respect to the filter having thepredetermined material and characteristics. Moreover, the derivedpressure loss indicates a minimum value with a specific cell density ina predetermined partition wall thickness as shown in FIG. 7. Therefore,during the manufacturing of the filter, the partition wall thickness andthe cell density can be determined in an appropriate range in which thepressure loss drops based on the value of the pressure loss calculatedwith respect to the filter having the specific material and shape. Thecell density and the partition wall thickness may be determinedbeforehand, and the material and the specific characteristics of thefilter may also be determined based on the calculated value of thepressure loss.

In the third aspect of the present invention, the shape of the filter ispreferably determined in consideration of the reduction of thetemperature gradient in the filter. For example, in an example of DPF,the temperature rise and the temperature gradient in the DPF by theburning of the PM at the time of regeneration depend on a thermalcapacity of the DPF. Therefore, when the partition wall thickness andthe cell density are set to appropriate values, an excessive rise oftemperature in the DPF can be prevented, and the temperature gradient inthe DPF can be reduced. The relation among the partition wall thickness,the cell density, and the temperature gradient in the filter can bemeasured, for example, when the DPFs having various partition wallthicknesses and cell densities are prepared, and actually attached tothe exhaust pipe of the diesel engine. In this case, when apredetermined amount of PM is deposited, the exhaust gas temperature israised to burn the PM deposited in the DPF.

When the partition wall thickness and the cell density in appropriateranges with respect to the pressure loss, preferably further thepartition wall thickness and the cell density in appropriate ranges withrespect to the temperature gradient are derived, the partition wallthickness and the cell density in optimum ranges with respect to boththe pressure loss and the temperature gradient can be derived. Forexample, a filter having the partition wall thickness and the celldensity in the optimum ranges can be manufactured which is usable in theexhaust emission control system according to the first aspect of thepresent invention.

Next, the exhaust emission control system according to the first aspectof the present invention will be described.

The exhaust emission control system of the first aspect is a systemincluding: an internal combustion engine 50; a filter 1 which capturesPM in an exhaust gas exhausted from the internal combustion engine 50;and an exhaust pipe 52 which introduces the exhaust gas exhausted fromthe internal combustion engine 50 into the filter 1 as shown in FIG. 3.The system further includes a combustion means or device whichintermittently burns the PM captured by the filter. Moreover, as shownin FIGS. 2(a), (b), the filter 1 includes at least two end faces 42 and44, porous partition walls 2 extending from one end face 42 to the otherend face 44, and through channels 3 a and 3 b partitioned by thepartition walls 2 and extending from one end face 42 through the otherend face 44, the predetermined through channels 3 a are plugged in oneend face 42, and the remaining predetermined through channels 3 b areplugged in the other end face 44 (not shown).

An oxidation catalyst 54 is preferably disposed in a stage before thefilter for a purpose of burning unburned carbohydrate and carbonmonoxide exhausted in a diesel exhaust gas in the stage before thefilter. When the oxidation catalyst is disposed, and when, for example,a fuel injection time is adjusted to raise the exhaust gas temperature,unburned contents (carbohydrate, carbon monoxide, etc.) in the exhaustgas burn by this oxidation catalyst, and a reaction heat is produced.Therefore, the catalyst is advantageously disposed, but this is not anessential requirement for the first aspect of the present invention, andthe catalyst does not have to be especially disposed.

By this constitution, the exhaust gas flowing into the filter from theinternal combustion engine flows into the through channels 3 b opened inthe end face 42, passes through the porous partition walls 2, and isexhausted from the through channels 3 a opened in the end face 44. Inthis case, the partition walls 2 constitute the filter, the PM and thelike exhausted, for example, from the diesel engine can be captured andremoved in the filter, the captured PM is intermittently burned by thecombustion means or device, and the filter can be periodicallyregenerated.

In important characteristics of the first aspect of the presentinvention, assuming that a partition wall thickness is (X) μm, and thecell density is (Y) cells/cm², X and Y fall: within a range of straightlines connecting points A1 (X is 267, Y is 50.4), B1 (X is 343, Y is27.1), C1 (X is 470, Y is 27.1), and D1 (X is 394, Y is 50.4) in thisorder in FIG. 1; preferably within a range surrounded by straight linesconnecting points A2 (X is 305, Y is 46.5), B2 (X is 356, Y is 31.0), C2(X is 432, Y is 31.0), and D2 (X is 381, Y is 46.5) in this order inFIG. 1; and further preferably within a range surrounded by straightlines connecting points A3 (X is 330, Y is 42.7), B3 (X is 356, Y is34.9), C3 (X is 406, Y is 34.9), and D3 (X is 381, Y is 42.7) in thisorder in FIG. 1.

This range is a range obtained by calculating the pressure lossaccording to the second aspect of the present invention, determining thecell density and the partition wall thickness in the appropriate rangesin which the pressure loss is reduced according to the third aspect, andfurther considering that a maximum temperature gradient in the filter bereduced. With the partition wall thickness and the cell density fallingin this range, an exhaust emission control system can be achieved inwhich the pressure loss is small and the temperature gradient in thefilter generated at the time of the burning of the PM captured by thefilter is small.

As described above, the larger the partition wall thickness is and thelarger the cell density is, the temperature gradient is reduced. Withrespect to the pressure loss, the cell density has a relation with afilter area. The larger the cell density is, the filter area increases,and the pressure loss can be reduced. The partition wall thickness alsoinfluences the pressure loss at a time when the gas passes through thepartition walls. The smaller the partition wall thickness is, thepressure loss is reduced. In this manner, the partition wall thicknessand the cell density have complicated influences on the pressure lossand the temperature behavior at the time of the filter regeneration, andit has been difficult to establish both the characteristics. However, bythe present invention, the partition wall thickness and the cell densityoptimum for the pressure loss and the temperature gradient can bederived.

The internal combustion engine in the first aspect of the presentinvention is not especially restricted as long as the internalcombustion engine contains the PM to be purified in the exhaust gas, butis preferably a diesel engine which contains a large amount ofparticulate matters. Examples of a means or device for intermittentlyburning the PM captured by the filter include a means or device forraising the temperature of the filter by a heater, a means or deviceusing a burner, and an exhaust gas temperature raising means or devicefor raising the exhaust gas temperature, and the means or device is notespecially restricted. However, the exhaust gas temperature raisingmeans or device is especially preferable. In this method, there is atendency that the temperature gradient generated in the filterincreases, and the method can be effectively applied to the first aspectof the present invention.

As the exhaust gas temperature raising means or device, the exhaust gastemperature is preferably raised, for example, by an adjustment devicewhich adjusts the supply time of the fuel to the internal combustionengine. For example, in the internal combustion engine including a fuelinjection device, the fuel injection time can be comparatively easilychanged. When the PM in the filter reaches a predetermined amount, theinjection time of the fuel can be changed to raise the exhaust gastemperature. An injection device which injects a part of the fuel isalso preferably disposed in the exhaust pipe, accordingly the burningoccurs in the exhaust pipe, and the exhaust gas temperature can beraised.

The exhaust emission control system of the first aspect of the presentinvention preferably includes a means or device which lowers the burningtemperature of the PM deposited on the filter in that the burning can beeasily performed, and the exhaust gas temperature raising means ordevice is especially preferably combined and used. This is because theexhaust gas temperature raising means or device does not easily raisethe temperature at a high temperature as compared with the combustionmeans or device of the PM by the heater or the like, and it is thereforeeffective to lower the burning temperature. As the means or device whichlowers the burning temperature, for example, an auxiliary agent addingdevice or the like is preferable, accordingly an auxiliary agent can beadded into the fuel quantitatively. When the fuel is burnt in acylinder, the added auxiliary agent is taken into the PM, and capturedin the DPF, and the burning temperature of the PM can be lowered by acatalytic function of the auxiliary agent. In the first aspect of thepresent invention, the means or device which lowers the burningtemperature of the PM is not restricted to only the auxiliary agentadding device, and may include any means or device that lowers theburning temperature.

Moreover, the exhaust emission control system of the first aspect of thepresent invention preferably includes a means, matter, or device whichpromotes the burning of the PM in that the burning can be easilyperformed. The means, matter, or device which promotes the burning ofthe PM is not especially restricted, and may include any promotingmeans, matter, or device. However, for example, it is preferable toapply the catalysts which promote the burning of the PM, such as Pt, Pd,and Rh, to the filter.

In the first aspect of the present invention, materials constituting thefilter are not especially restricted, but various ceramic materialscontaining oxide or non-oxide as a main component are preferable fromviewpoints of strength, heat resistance, and durability. Concreteexamples are considered to include cordierite, mullite, alumina, spinel,silicon carbide, silicon nitride, lithium aluminum silicate, aluminumtitanate, and the like. One or two or more selected from them arepreferably used as the main components, and especially cordierite,silicon carbide, or a silicon-silicon carbide based material ispreferable. Here, the “main component” means that the componentconstitutes 50% by mass or more, preferably 70% by mass or more, furtherpreferably 80% by mass or more of the filter.

In the first aspect of the present invention, the filter is preferably afilter in which a plurality of segments are integrated, or a filterincluding slits. When the filter is divided into a plurality ofsegments, and the segments are integrated, or the slits are formed inthe filter, a thermal stress is scattered, and cracks by the thermalstress can be prevented. When the filter is segmented and integrated, asize or shape of each segment is not restricted. However, when eachsegment is excessively large, an effect of preventing the cracks bysegmentation is not sufficiently fulfilled. When the segment isexcessively small, the manufacturing of the respective segments or theintegrating thereof by bonding unfavorably becomes complicated. In apreferable size of the segment, a sectional area is 900 to 10000 mm²,further preferably 900 to 5000 mm², and most preferably 900 to 3600 mm2,and 70% by volume or more of the filter is preferably constituted of thehoneycomb segment having this size. In a preferable shape of thesegment, for example, a quadrangular sectional shape, that is, thesegment having a square pole shape is regarded as a basic shape, and theshape of the segment on an outer peripheral side can be appropriatelyselected in accordance with the shape of the integrated filter. Thewhole sectional shape of the filter is not especially restricted, and isnot limited to a circular shape shown in FIG. 2(a), and, for example, inaddition to an elliptic shape, a race track shape, a substantiallycircular shape such as an oblong shape, a quadrangular shape, and apolygonal shape such as a hexagonal shape may also be used.

The partition walls of the filter in the first aspect of the presentinvention are porous, but a pore diameter and a porosity of thepartition wall are not especially restricted, and can be appropriatelyselected in accordance with the application by any person skilled in theart. In general, the pore diameter can be selected in accordance with aparticle diameter of the PM or the like. For example, when the partitionwalls are used in the DPF, the average pore diameter is preferably setto about 5 to 70 μm, further preferably about 10 to 50 μm, especiallypreferably about 15 to 30 μm. The porosity can be similarlyappropriately selected in accordance with the application. When theporosity is excessively small, an initial pressure loss is excessivelylarge. When the porosity is excessively large, strength unfavorablybecomes insufficient. For example, the preferable porosity for use inthe DPF is in a range of 30 to 90%. When the porosity is less than 30%,the pressure loss becomes excessively large. When the porosity exceeds90%, the strength of the ceramic material runs short. The thermalconductivity of the filter is not especially restricted, but ispreferably 8 to 70 W/mK, further preferably 10 to 55 W/mK.

The method of manufacturing the filter is not especially restricted, butthe filter can be manufactured, for example, by the following method.

A material selected from the above-described preferable materials, forexample, a silicon carbide powder is used as a raw material powder ofthe filter, binders such as methyl cellulose and hydroxypropoxyl methylcellulose are added to the powder, and further surfactant and water areadded to obtain plastic clay. Moreover, for example, the partition wallthickness and the cell density in the appropriate ranges are determinedaccording to the third aspect of the present invention, an extruderincluding a ferrule which forms the shape is used, the obtained clay isextrusion-molded, and accordingly a molded article having a honeycombstructure is obtained. This article is dried, for example, by microwavesand hot air, and thereafter one-end portions of adjacent throughchannels on opposite sides are plugged with a material similar to thatused in manufacturing the filter. The article is further dried,heated/degreased, for example, in a nitrogen atmosphere, and thereaftercalcined in an inactive atmosphere such as argon so that the filter canbe obtained. A calcining temperature and a calcining atmosphere differwith the raw material, and any person skilled in the art can select thecalcining temperature and calcining atmosphere which are appropriate forthe selected ceramic raw material.

To form the filter into a constitution in which a plurality of segmentsare integrated, after obtaining the segments in the above-describedmethod, the obtained segments are bonded, for example, using ceramiccement, and dried/hardened so that the filter can be obtained. A methodof imparting the catalyst to the filter manufactured in this manner maybe a method usually performed by the person skilled in the art. Forexample, when a catalyst slurry is wash-coated, dried, and calcined, thecatalyst can be supported.

The present invention will be described hereinafter concretely inaccordance with examples, but the present invention is not limited toany of these examples.

EXAMPLES AND COMPARATIVE EXAMPLES

Next, the present invention will be described further concretely basedon the examples.

Example 1

A filter having the same characteristics as those of Filter A used inthe description of the second aspect of the present invention and havinga cell density of 46.5 cells/cm², and a partition wall thickness of 305μm was prepared. The filter was attached to a diesel engine in aconstitution shown in FIG. 3, a relation between a deposited amount ofPM and a pressure loss was actually measured on conditions of a gasamount of 2.27 Nm³/min. and an inflow gas temperature at 200° C., andactual measurement was compared with a calculated value calculated usingEquation (8). Results are shown in FIG. 8, and the actual measurementmatched the calculated value very well.

Examples 2 to 6 and Comparative Examples 1, 2

Filters having the same filter characteristics as those of Filter A andhaving cell densities and partition wall thicknesses shown in Table 1were prepared. The filter was attached to a system including a 2.0 literdiesel engine and an auxiliary agent adding device in a basicconstitution shown in FIG. 3 to constitute an exhaust emission controlsystem. The auxiliary agent adding device was used, 25 ppm of a Ce fuelauxiliary agent was added to fuel, and the system was operated. When aPM deposited amount per unit filter volume reached 6 kg/m³, the exhaustgas temperature was raised at 600° C. to burn the PM, regeneration ofthe filter was evaluated, and a maximum temperature gradient in eachsegment was measured. Results are shown in Table 1 and FIG. 9. TABLE 1Partition Maximum wall Cell density, temperature thickness μ cells/cm²gradient Pressure mil indicated cells/in² indicated in segment loss,within ( ) within ( ) ° C./cm kPa Example 2 305 (12) 46.5 (300) 170 70.7Example 3 381 (15) 46.5 (300) 130 79.9 Example 4 381 (15) 38.8 (250) 12578.8 Example 5 381 (15) 31.0 (200) 144 80.4 Example 6 368 (14.5) 38.8(250) 131 77.4 Comparative 457 (18) 46.5 (300)  78 91.7 Example 1Comparative 254 (10) 46.5 (300) 210 (cracks 65.6 Example 2 generated inDPF)Regeneration test:

-   DPF shape: φ5.66″×6″L-   Soot deposited amount: 6 g/L    Pressure Loss Calculation/Actual Measurement Conditions:-   DPF shape: φ5.66″×10″L

PM deposited amount: 5 g/L

-   Gas amount: 10 NM3/min.-   Gas temperature: 665° C.

From the results, it is seen that the larger the partition wallthickness is, a thermal capacity increases, therefore a maximumtemperature in the filter at the time of regeneration drops, andadditionally a temperature gradient in the filter is reduced. It is alsoseen that the larger the cell density is, the thermal capacityincreases, and the temperature gradient in the segment is also reduced.A temperature gradient at the time of the filter regeneration, at whichany crack was not generated, was 180° C./cm.

Values of the pressure loss calculated using Equation (8) based on thefilter shape and characteristics are shown in Table 1. The maximumtemperature gradients and pressure losses of samples of Examples 2 to 6fell in satisfactory ranges, but the pressure loss of the sample ofComparative Example 1 was excessively large, the maximum temperaturegradient of the sample of Comparative Example 2 was excessively large,and the filters were cracked.

Industrial Applicability

As described above, according to a first aspect of the presentinvention, an exhaust emission control system is a satisfactory systemin which a pressure loss at the time of PM deposition is small, amaximum temperature gradient in a filter at the time of filterregeneration by PM burning is small, and the filter is not easilycracked. In a value obtained by a method of calculating a pressure lossaccording to a second aspect of the present invention, satisfactoryagreement with actual measurement is seen, and the pressure loss can bemeasured with good accuracy. According to a third aspect of the presentinvention, a filter which falls in the scope of the first aspect of thepresent invention can be easily prepared according to a third aspect ofthe present invention.

1-14. (canceled)
 15. An exhaust emission control system comprising: aninternal combustion engine; a filter for capturing particulate mattersin an exhaust gas exhausted from the internal combustion engine; and anexhaust pipe for introducing the exhaust gas into the filter, the systemcomprising combustion means for intermittently burning the particulatematters captured by the filter, the filter being a honeycomb filterincluding: at least two end faces; porous partition walls extending fromone end face to the other end face; and a large number of throughchannels partitioned by the partition walls and extending from one endface through the other end face, predetermined through channels beingsealed in one end face, remaining predetermined through channels' beingsealed in the other end face, wherein assuming that a partition wallthickness is (X) μm, and the number of through channels per unit area ina section vertical to a longitudinal direction of the through channelsis (Y) cells/cm², X and Y fall within a range surrounded by straightlines connecting points A3 (X is 330, Y is 42.7), B3 (X is 356, Y is34.9), C3 (X is 406, Y is 34.9), and D3 (X is 381, Y is 42.7) in thisorder in FIG.
 1. 16. The exhaust emission control system according toclaim 15, wherein the internal combustion engine is a diesel engine. 17.The exhaust emission control system according to claim 15, wherein thecombustion means includes exhaust gas temperature raising means forraising a temperature of the exhaust gas in such a manner as to startthe burning of the particulate matters captured by the filter.
 18. Theexhaust emission control system according to claim 17, wherein theexhaust gas temperature raising means includes an adjustment device foradjusting a time to supply fuel to the internal combustion engine. 19.The exhaust emission control system according to claim 17, wherein theexhaust gas temperature raising means includes a supply device forsupplying the fuel into the exhaust pipe.
 20. The exhaust emissioncontrol system according to claim 15, further comprising: means forlowering the burning temperature of the particulate matters captured bythe filter.
 21. The exhaust emission control system according to claim15, further comprising: means for promoting the burning of theparticulate matters captured by the filter.
 22. The exhaust emissioncontrol system according to claim 15, wherein the filter contains aceramic material as a main component.
 23. The exhaust emission controlsystem according to claim 15, wherein the filter is constituted byintegration of a plurality of segments of a honeycomb structure.
 24. Amethod of calculating a pressure loss of a honeycomb filter including:at least two end faces; porous partition walls extending from one endface to the other end face; and a large number of through channelspartitioned by the partition walls and extending from one end facethrough the other end face, predetermined through channels being pluggedin one end face, remaining predetermined through channels being pluggedin the other end face, the method comprising the steps of: decomposingthe pressure loss into at least a pressure loss in a plugged portion, apressure loss in the through channel, and a pressure loss in thepartition wall; and decomposing the pressure loss in the partition wallinto pressure losses in cases where any particulate matter is notdeposited in the filter and where the particulate matters are depositedto calculate the pressure loss.
 25. The method of calculating thepressure loss according to claim 24, further comprising the steps of:measuring the pressure loss in the case where the particulate mattersare deposited in the predetermined filter; and calculating the pressureloss in the partition wall in the case where the particulate matters aredeposited in the filter based on an equation obtained by curve fittingof an increase behavior of the obtained pressure loss.
 26. A method ofmanufacturing a filter, wherein a shape of the filter is determined byuse of a pressure loss value obtained by the calculation methodaccording to claim
 24. 27. A method of manufacturing a filter, wherein ashape of the filter is determined by use of a pressure loss valueobtained by the calculation method according to claim 25.